In a memory system using a NAND type flash memory (which will be referred to as “NAND memory”, hereinafter) as a storage medium, a threshold voltage of a memory cell might change because of, for example, data retention and/or read disturb, whereby a bit error rate of data read from the NAND memory is deteriorated, and thus the reliability of the memory system is lowered. Methods for improving the reliability thus lowered may include a shift read that executes a read operation by using a read voltage level shifted from a read voltage level used for a normal read operation (which will also be referred to as “normal read”, hereinafter). Hereinafter, a read voltage level will be simply referred to as “read voltage”.